The present invention relates generally to the field of pressure sensors and transducers. More particularly, it relates to a capacitive pressure sensor of the type in which the capacitive plates are formed from wafers of conductive silicon, at least one of which is flexed in response to an applied pressure, whereby the capacitance is varied as a function of the applied pressure.
Capacitive pressure sensors are becoming increasingly popular for a number of applications due to their high pressure sensitivity and relatively low temperature sensitivity, as compared to, for example, pressure sensors of the piezoresistive type. While a number of types of capacitive pressure sensors have been developed, the increasing needs for lower cost, simpler structure, and smaller size have resulted in growing interest in the silicon capacitive pressure sensor.
In a capacitive pressure sensor, a pressure-responsive diaphragm forms one of the two plates of a capacitor. Pressure-induced deflection of the diaphragm varies the distance between the two plates, thereby changing the capacitance of the sensor. The change in capacitance, in turn, creates a change in an electrical output signal of the circuit which contains the sensor. For example, the change in capacitance can create a frequency shift which can be translated, by suitable circuitry, into a voltage that is a function, ultimately, of the applied pressure.
In the silicon capacitive pressure sensor, the plates of the capacitor are formed by wafers of silicon, appropriately doped to achieve a suitable degree of conductivity. One (or, possibly, both) of the wafers is configured into a pressure-responsive diaphragm. Spacing between the wafers is maintained by a suitable insulating material, usually glass, to which the wafers are joined by a variety of means, e.g., electrostatic or anodic bonding, glass frit, and metal film brazing. Examples of such devices are disclosed in the following U.S. patents:
U.S. Pat. No. 3,634,727--Polye
U.S. Pat. No. 4,405,970--Swindal et al.
U.S. Pat. No. 4,415,948--Grantham et al.
Of increasing importance in silicon capacitive pressure sensors are the needs to lower the costs and reduce the size of the sensors. Both goals can be achieved through batch-processing techniques. Until fairly recently, however, batch-processing techniques have been applied primarily to piezoresistive pressure sensors and to capacitive pressure sensors of the type employing a conductive silicon diaphragm bonded to a glass or ceramic substrate, with a metallized surface on the substrate providing one capacitor plate, and the diaphragm providing the other. Examples of such "silicon-on-glass" batch-processed capacitive pressure sensors are disclosed in U.S. Pat. No. 4,386,453 to Giachino et al., and in Lee et al., "A Batch-Fabricated Silicon Capacitive Pressure Sensor with Low Temperature Sensitivity", IEEE Transactions on Electron Devices, Vol. ED-29, No. 1, January, 1982 (pp. 42-48). The "silicon-on-glass" technology, however, does have some drawbacks. For example, an electrical connection must be provided through the substrate to the metallization on its inner surface that forms one capacitive plate, thereby adding to the complexity of the fabrication process. Also, the substrate material must be carefully selected to provide both good dielectric qualities and close thermal matching to the silicon diaphragm. Thus, a borosilicate glass is usually selected, its coefficient of thermal expansion being workably close, although not identical, to that of silicon.
The above-referenced patents to Swindal et al. and to Grantham et al. disclose recently-developed methods for batch-process fabrication of capacitive pressure sensors employing silicon wafers for both of the capacitor plates. The sensors made by these methods are relatively low in cost and small in size (i.e., approximately 0.5 cm in diameter), with the use of conductive silicon as the capacitor plates allowing external electrical connection to the plates to be made simply by metallizing the external surfaces of the silicon wafers. In these sensors, spacing between the plates is provided by borosilicate glass structures formed on one of the wafers, with the second, pressure-responsive silicon wafer being bonded to the borosilicate glass by "field-assisted" (electrostatic) bonding.
While the sensors and fabrication methods disclosed in the aforementioned patents to Swindal et al. and Grantham et al. offer some advantages in terms of low cost and small size, futher improvements in the technology have been sought. For example, as the diameter of the capacitor plates decreases, the total capacitance of the sensor decreases proportionately, unless the plates are brought closer together. In the prior art, as exemplified by the Swindal et al. and Grantham et al. patents, the spacing between the plates is on the order of 2 to 3 microns. To provide either substantially greater capacitance or substantially smaller plate diameter (either or both of which may be desirable in certain applications), it is necessary to decrease this spacing by as much as an order of magnitude.
As the spacing between the plates decreases, the need to control the spacing with precision increases significantly. In the prior art, this is accomplished by a borosilicate glass layer deposited on one of the silicon wafers. Thus, the precision of the spacing between the plates is dependent upon the precision with which the thickness of the deposited glass layer can be controlled. According to the aforementioned patent to Swindal et al. this precision is .+-.5%. It would thus be an improvement in the art to achieve at least comparable precision in plate spacing without the additional complexity of a precision glass sputtering process.
Another limitation in the prior art is the use of borosilicate glass as the spacing medium between the two silicon plates. Borosilicate glass is used because the relative thickness of the spacing medium necessitates a material having a coefficient of thermal expansion that is close to that of monocrystalline silicon, in order to prevent the development of structural flaws when the sensor is subjected to widely varying temperatures. Borosilicate glass, however, is prone to a deterioration in structural integrity at temperatures in excess of about 500.degree. C., making such sensors unsuited to many high-temperature applications.
Thus, it can be seen that there is a need for a batch-processing method for fabricating silicon capacitive pressure sensors that yields sensors with small physical dimensions and acceptable capacitance levels, while maintaining low cost and precise dimensional control. The sensors so made should, preferably, be suitable for use in high-temperature environments, and in environments where drastic temperature changes may be experienced.